Electrophoretic display panel

ABSTRACT

An electrophoretic display panel ( 1 ), for displaying pictures having a plurality of picture elements, has a plurality of pixels ( 2 ) and drive means ( 100 ). The pixels ( 2 ) have a first electrode ( 3 ) and a second electrode ( 4 ) for receiving a potential difference, and an electrophoretic medium ( 5 ) which is present between the first electrode ( 3 ) and the second electrode ( 4 ). The medium ( 5 ) has a first and a second extreme optical state and an intermediate optical state, intermediate between the first and the second extreme optical state. For the display panel ( 1 ) to be able to display a reproducible intermediate optical state, the drive means ( 100 ) are able to control, in operation, the potential difference for changing the optical state between the first extreme, the second extreme and a singular equilibrium optical state as the intermediate optical state, in dependence of the picture element to be displayed.

The invention relates to an electrophoretic display panel, fordisplaying pictures comprising a plurality of picture elements,comprising:

-   -   a plurality of pixels for displaying the picture elements, each        pixel comprising:        -   a first electrode and a second electrode for receiving a            potential difference; and        -   an electrophoretic medium between the first electrode and            the second electrode, which medium has a first and a second            extreme optical state and an intermediate optical state            intermediate between the first and the second extreme            optical state; and    -   drive means able to control, in operation, the potential        difference, having a pulse duration, for changing the optical        state between the first extreme, the second extreme and the        intermediate optical state, in dependence of the picture element        to be displayed.

An embodiment of the electrophoretic display panel of the type mentionedin the opening paragraph is described in non-published European Patentapplication 02075846.2 (PHNL 020156).

In the described electrophoretic display panel each pixel represents onepicture element. The optical state of the pixel equals the optical stateof the represented picture element. The electrophoretic medium of thepixel includes positively and negatively charged particles in atransparent liquid. The positively charged particles have a colordifferent from the negatively charged particles. In operation, thepotential difference, controlled by the drive means, determines themotion of the charged particles. If the positively charged particles arepositioned at the first electrode and the negatively charged particlesare positioned at the second electrode, the medium is in the firstextreme optical state. At the side of the first electrode, the pictureelement has the color of the positively charged particles. At anopposite potential difference the charged particles are at oppositepositions and the medium is in the second extreme optical state. At theside of the first electrode, the picture element has the color of thenegatively charged particles. To change the optical state of the mediumfrom the first extreme optical state into the second extreme opticalstate and vice versa, the potential difference is relatively large andthe pulse duration is relatively long. The optical state reached isinsensitive to an overshoot in the potential difference and/or the pulseduration as a larger potential difference and/or a longer pulse durationhas no further effect on the optical state. The display panel is able todisplay intermediate optical states, referred to as gray values. Grayvalue is herein understood to mean a color value in between the color ofthe first and the second extreme optical state. If the first and thesecond optical state represent white and black, the gray valuerepresents a shade of gray; if the first and second optical staterepresent two other colors, the gray value stands for a mixed color ofthese two colors. In operation, to display the gray value the potentialdifference is pulsed, controlled by the drive means, wherein the pulseduration, the potential difference and many factors, that are difficultto control, determine the gray value. If, for instance, the viscosity orthe dielectric constant of the liquid and/or particles changes due toe.g. a temperature variation, the motion of the charged particles ismodified and the same pulse duration and the same potential differenceresults in a different gray value. Therefore, it is difficult to displaya gray value in a reproducible manner.

It is a drawback of the described display panel that it is difficult toobtain therewith a reproducible gray value in the displayed picture.

It is an object of the invention to provide a display panel of the kindmentioned in the opening paragraph which is able to display, inoperation, a reproducible gray value.

The object is thereby achieved that the drive means are able to controla singular equilibrium optical state as the intermediate optical state.

The invention is based on the insight that if the drive means are ableto control the gray value representing an equilibrium optical state asthe intermediate state to obtain the gray value thereof, the dependencyof the relationship between this gray value and the many factors thatare difficult to control is reduced. For instance, the temperaturedependency depends on rheological properties of the particles within theliquid; such temperature dependency is much smaller, since therheological properties are much less important. It has been observedthat the electrophoretic medium reaches the same gray value, the grayvalue of 30 the equilibrium optical state, if the potential differenceis substantially zero. Therefore, the display panel is able to display,in operation, a reproducible gray value.

The time interval to reach the equilibrium gray value is, for instance,tens of seconds to tens of minutes. The displayed picture is changedfaster, if the drive means are able to control the potential difference:

-   -   of equal sign and a relative short pulse duration for changing        the optical state from the first optical state to the        equilibrium optical state, as compared to the potential        difference and the pulse duration for changing the optical state        from the first to the second optical state, and    -   of equal sign and a relative short pulse duration for changing        the optical state from the second optical state to the        equilibrium optical state, as compared to the potential        difference and the pulse duration for changing the optical state        from the second to the first optical state, and    -   subsequently being substantially zero.        The potential difference having equal sign and relative short        pulse duration brings the gray value near the equilibrium gray        value. Subsequently, the potential difference is substantially        zero and the electrophoretic medium reaches the equilibrium gray        value.

The gray value depends on the number and the size of the coloredparticles. The displayed picture has a relatively good picture quality,if the equilibrium optical state is in the middle of the first and thesecond extreme optical state. Then the gray value is mid gray. Inpractice, the equilibrium optical state represents about mid gray, iffor instance the number and the size of the positively charged particlesis close to the number and the size of the negatively charged particles.

If each picture element is represented by one pixel, each pictureelement is able to have three optical states. However, if the drivemeans are able to represent each picture element by at least twoneighboring pixels, each picture element is able to have more than threeoptical states, because of the optical states that are formed bycombinations of optical states of the at least two neighboring pixels.If, furthermore, the equilibrium optical states are in the middle of thefirst and the second extreme optical states, the displayed picture hasan even better picture quality. If, furthermore, the at least twoneighboring pixels each have a surface with an area for displaying theoptical state, a first area of the areas being substantially ⅓ of asecond area of the areas, the picture element has at least nine,substantially uniformly distributed, optical states, between the twoextreme optical states.

These and other aspects of the invention will be further elucidated anddescribed with reference to the drawings, in which:

FIG. 1 shows diagrammatically a front view of the display panel,

FIG. 2 shows diagrammatically a cross-sectional view along II-II in FIG.1,

FIG. 3 shows diagrammatically a front view of the display panel, and

FIG. 4 shows diagrammatically an equivalent circuit diagram of a portionof the display panel.

The Figures are schematic and not drawn to scale and in all the figuressame reference numerals refer to corresponding parts.

In FIG. 1 the display panel 1 has pixels 2. The pixels 2 are forinstance arranged along substantially straight lines in atwo-dimensional structure. For instance, one pixel represents onepicture element.

In FIG. 2 the pixel 2 has a first electrode 3 and a second electrode 4,present on substrates 9, for receiving a potential difference.Furthermore, an electrophoretic medium 5 is present between the firstelectrode 3 and the second electrode 4, for instance positively chargedblack particles 6 and negatively charged white particles 7 in atransparent liquid. If the positively charged particles 6 are positionedat the first electrode 3 and the negatively charged particles 7 arepositioned at the second electrode 4, the electrophoretic medium 5 is inthe first extreme optical state. The potential difference is forinstance −5 Volts. If the picture element is viewed from the side of thefirst electrode 3, the first electrode 3 being transparent, the pictureelement is black. In a reversal position of the charged particles 6,7,the electrophoretic medium 5 is in the second extreme optical state, andthe picture element, viewed from the side of the first electrode 3, iswhite. Then the potential difference is for instance 5 Volts. To changethe optical state of the electrophoretic medium 5 into one of theextreme optical states, the drive means are able to apply e.g. apotential difference of −5 Volts applied for a period of 5 seconds tochange the optical state into the first extreme optical state and apotential difference of 5 Volts applied for a period of 5 seconds tochange the optical state into the second extreme optical state.

If the electrophoretic medium 5 is in one of the extreme optical statesand the potential difference is changed by the drive means to 0 Volts,the optical state changes slowly towards the equilibrium optical state,intermediate the extreme optical states, in the example the gray value.The interval to reach the intermediate optical state is adjustable,typically varying from tens of seconds to tens of minutes. This intervalis shorter, for instance 2 seconds, if a potential difference of,

5 Volts is applied for 1 second for changing the optical state from thefirst optical state to the equilibrium optical state, and

−5 Volts is applied for 1 second for changing the optical state from thesecond optical state to the equilibrium optical state, and,subsequently, the potential difference is substantially zero. Then theelectrophoretic medium reaches the equilibrium gray value.

If the number and sizes of the positively and the negatively chargedparticles 6,7 are equal, the equilibrium optical state represents midgray and the displayed picture has a relatively good picture quality. Ifthe size of the positively charged particles 6 is larger than the sizeof the negatively charged particles 7 the equilibrium optical staterepresents a color nearer to the color of the positively chargedparticles 6 than to the color of the negatively charged particles 7.

In FIG. 3 the two neighboring pixels 2′ representing a picture elementare indicated. The optical state of each of the neighboring pixels 2′ isdefined as 0, being the first extreme optical state, 1, being theequilibrium optical state, and 2, being the second extreme opticalstate. Then the picture element has nine optical states:

-   -   00: both neighboring pixels 2′ in optical state 0,    -   01: the first neighboring pixel 2′ in optical state 0 and the        second neighboring pixel 2′ in optical state 1,    -   02, 10, 11, 12, 20, 21, and 22.        As an example, the two neighboring pixels 2′ each have their        equilibrium optical state in the middle of the first and the        second extreme optical state. Furthermore, the area for        displaying the optical state of the first neighboring pixel 2′        is substantially three times the area for displaying the optical        state of the second neighboring pixel 2′. The represented        picture element has nine, substantially uniformly distributed,        optical states, between the two extreme optical states. For        three neighboring pixels representing the picture element, the        area for displaying the optical state of the first neighboring        pixel being about three times the area for displaying the        optical state of the second neighboring pixel and the area for        displaying the optical state of the second neighboring pixel        being about three times the area for displaying the optical        state of the third neighboring pixel, the picture element has        27, substantially uniformly distributed, optical states, between        the two extreme optical states, etc.

An electric equivalent, shown diagrammatically in FIG. 4, of a portionof the display panel 1 to which the invention is applicable, comprisesdrive means 100 and a matrix of pixels 2 at the area of crossings of rowor selection electrodes 70 and column or data electrodes 60. The rowelectrodes 70 numbered from 1 to m in FIG. 4 are consecutively selectedby means of a row driver 40, while the column electrodes 60 numberedfrom 1 to n in FIG. 4 are provided with data via a data register 50. Ifnecessary, data to be displayed 20 is first processed in a processor 30.Mutual synchronization between the row driver 40 and the data register50 takes place via drive lines 80 connected to the processor 30. Thedrive means 100 comprise, for example, the row driver 40, the rowelectrodes 70, the data register 50, the column electrodes 60, the drivelines 80 and the processor 30.

Drive signals from the row driver 40 and the data register 50 select apixel 2, referred to as passive drive. A column electrode 60 receivessuch a potential with respect to a row electrode 70 that the pixel 2obtains one of the extreme optical states or the equilibrium opticalstate at the area of the crossing, for example, black, white or midgray. Drive signals from the row driver 40 select the pixels 2 viathin-film transistors, denoted as TFTs, 90 whose gate electrodes areelectrically connected to the row electrodes 70 and whose sourceelectrodes are electrically connected to the column electrodes 60,referred to as active drive. The signal present at the column electrode60 is transferred via the TFT 90 to the pixel 2. In the example of FIG.4, such a TFT 90 is shown diagrammatically for only one pixel 2.

It will be apparent that within the scope of the invention manyvariations are possible for a person skilled in the art.

The scope of the invention is not limited to the exemplary embodimentsdescribed herein. The invention is embodied in every novel feature andevery combination of features.

1. An electrophoretic display panel, for displaying pictures comprisinga plurality of picture elements, comprising: a plurality of pixels fordisplaying the picture elements, each pixel comprising: a firstelectrode and a second electrode for receiving a potential difference;and an electrophoretic medium between the first electrode and the secondelectrode, which medium has a first and a second extreme optical stateand an intermediate optical state intermediate between the first and thesecond extreme optical state; and drive means able to control, inoperation, the potential difference, having a pulse duration, forchanging the optical state between the first extreme, the second extremeand the intermediate optical state, in dependence of the picture elementto be displayed, characterized in that the drive means are able tocontrol a singular equilibrium optical state as the intermediate opticalstate.
 2. An electrophoretic display panel as claimed in claimed 1characterized in that the drive means are able to control the potentialdifference: of equal sign and a relative short pulse duration forchanging the optical state from the first optical state to theequilibrium optical state, as compared to the potential difference andthe pulse duration for changing the optical state from the first to thesecond optical state, and of equal sign and a relative short pulseduration for changing the optical state from the second optical state tothe equilibrium optical state, as compared to the potential differenceand the pulse duration for changing the optical state from the second tothe first optical state, and subsequently being substantially zero. 3.An electrophoretic display panel as claimed in claimed 1 characterizedin that the equilibrium optical state is in the middle of the first andthe second extreme optical state.
 4. An electrophoretic display panel asclaimed in claim 1 characterized in that the drive means are able torepresent each picture element by at least two neighboring pixels.
 5. Anelectrophoretic display panel as claimed in claimed 4 characterized inthat the at least two neighboring pixels each have a surface with anarea for displaying the optical state, a first area of the areas beingsubstantially ⅓ of a second area of the areas.